Internet Engineering Task Force M. Baushke
Internet-Draft Juniper Networks, Inc.
Updates: 4253, 4419, 4432, 4462, 5656 March 8, 2016
(if approved)
Intended status: Standards Track
Expires: September 9, 2016

Key Exchange Method Updates for Secure Shell (SSH)


This document deprecates some previously specified Key Exchange Method algorithm names as well as defining a few added Modular Exponential (MODP) Groups for the Secure Shell (SSH) protocol. It also updates [RFC4253], [RFC4419], [RFC4462], and [RFC5656] by specifying the set key exchange algorithms that currently exist and which ones MUST, SHOULD, MAY, and SHOULD NOT be implemented. New key exchange methods use the SHA-2 family of hashes.

Status of This Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on September 9, 2016.

Copyright Notice

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1. Overview and Rationale

Secure Shell (SSH) is a common protocol for secure communication on the Internet. In [RFC4253], SSH originally defined the Key Exchange Method Name diffie-hellman-group1-sha1 which used [RFC2409] Oakley Group 1 (a MODP group with 768 bits) and SHA-1 [RFC3174]. Due to recent security concerns with SHA-1 [RFC6194] and with MODP groups with less than 2048 bits [NIST-SP-800-131Ar1] implementer and users request support for larger MODP group sizes with data integrity verification using the SHA-2 family of secure hash algorithms as well as MODP groups providing more security.

The United States Information Assurance Directorate at the National Security Agency has published a FAQ [MFQ-U-OO-815099-15] suggesting that the use of ECDH using the nistp256 curve and SHA-2 based hashes less than SHA2-384 are no longer sufficient for transport of Top Secret information. It is for this reason that this draft moves ecdh-sha2-nistp256 from a REQUIRED to OPTIONAL as a key exchange method. This is the same reason that the stronger MODP groups being introduced are using SHA2-512 as the hash algorithm. Group14 is already present in most SSH implementations and most implementations already have a SHA2-256 implementation, so diffie-hellman-group14-sha256 is provided as an easy to implement and faster to use key exchange for small embedded applications.

It has been observed in [safe-curves] that the NIST recommended Elliptic Curve Prime Curves (P-256, P-384, and P-521) are perhaps not the best available for Elliptic Curve Cryptography Security. For this reason, none of the [RFC5656] curves are marked as a MUST implement. However, the requirement that "every compliant SSH ECC implementation MUST implement ECDH key exchange" is now taken to mean that if ecdsa-sha2-[identifier] is implemented, then ecdh-sha2-[identifier] MUST be implemented.

Please send comments on this draft to

2. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

3. Key Exchange Algorithms

This memo adopts the style and conventions of [RFC4253] in specifying how the use of new data key exchange is indicated in SSH.

The following new key exchange algorithms are defined:

Key Exchange Method Name          Note
diffie-hellman-group14-sha256     MAY/OPTIONAL
diffie-hellman-group16-sha512     SHOULD/RECOMMENDED
diffie-hellman-group18-sha512     MAY/OPTIONAL

Figure 1

The SHA-2 family of secure hash algorithms are defined in [FIPS-180-4].

The method of key exchange used for the name "diffie-hellman-group14-sha256" is the same as that for "diffie-hellman-group14-sha1" except that the SHA2-256 hash algorithm is used.

The group16 and group18 names are the same as those specified in [RFC3526] 4096-bit MODP Group 16 and 8192-bit MODP Group 18.

The SHA2-512 algorithm is to be used when "sha512" is specified as a part of the key exchange method name.

4. IANA Considerations

This document augments the Key Exchange Method Names in [RFC4253]. It downgrades the use of SHA-1 hashing for key exchange methods in [RFC4419], [RFC4432], and [RFC4462]. It also moves from MUST to MAY the ecdh-sha2-nistp256 given in [RFC5656].

It is desirable to also include the ssh-curves from the [I-D.josefsson-ssh-curves] in this list. The "curve25519-sha256" is currently available in some Secure Shell implementations under the name "" and is the best candidate for a fast, safe, and secure key exchange method.

IANA is requested to update the SSH algorithm registry with the following entries:

Key Exchange Method Name              Reference     Note
diffie-hellman-group-exchange-sha1    RFC4419       SHOULD NOT
diffie-hellman-group-exchange-sha256  RFC4419       MAY
diffie-hellman-group1-sha1            RFC4253       SHOULD NOT
diffie-hellman-group14-sha1           RFC4253       SHOULD
ecdh-sha2-nistp256                    RFC5656       MAY
ecdh-sha2-nistp384                    RFC5656       SHOULD
ecdh-sha2-nistp521                    RFC5656       SHOULD
ecdh-sha2-*                           RFC5656       MAY
ecmqv-sha2                            RFC5656       MAY
gss-gex-sha1-*                        RFC4462       SHOULD NOT
gss-group1-sha1-*                     RFC4462       SHOULD NOT
gss-group14-sha1-*                    RFC4462       MAY
gss-*                                 RFC4462       MAY
rsa1024-sha1                          RFC4432       SHOULD NOT
rsa2048-sha256                        RFC4432       MAY
diffie-hellman-group14-sha256         This Draft    MAY
diffie-hellman-group16-sha512         This Draft    SHOULD
diffie-hellman-group18-sha512         This Draft    MAY
curve25519-sha256                     ssh-curves    MUST
curve448-sha512                       ssh-curves    MAY

Figure 2

The Note in the above table is an implementation suggestion/recommendation for the listed key exchange method. It is up to the end-user as to what algorithms they choose to be able to negotiate.

The guidance of his document is that the SHA-1 algorithm hashing SHOULD NOT be used. If it is used, it should only be provided for backwards compatibility, should not be used in new designs, and should be phased out of existing key exchanges as quickly as possible because of its known weaknesses. Any key exchange using SHA-1 SHOULD NOT be in a default key exchange list if at all possible. If they are needed for backward compatibility, they SHOULD be listed after all of the SHA-2 based key exchanges.

The RFC4253 REQUIRED diffie-hellman-group14-sha1 method SHOULD be retained for compatibility with older Secure Shell implementations. It is intended that this key exchange be phased out as soon as possible.

5. Acknowledgements

Thanks to the following people for review and comments: Denis Bider, Peter Gutmann, Damien Miller, Niels Moeller, Matt Johnston, Iwamoto Kouichi, Simon Josefsson, Dave Dugal.

Thanks to the following people for code to implement interoperable exchanges using some of these groups as found in an the -01 draft: Darren Tucker for OpenSSH and Matt Johnston for Dropbear. And thanks to Iwamoto Kouichi for information about RLogin, Tera Term (ttssh) and Poderosa implementations also adopting new Diffie-Hellman groups based on the -01 draft.

6. Security Considerations

The security considerations of [RFC4253] apply to this document.

The security considerations of [RFC3526] suggest that these MODP groups have security strengths given in this table. They are based on [RFC3766] Determining Strengths For Public Keys Used For Exchanging Symmetric Keys.

Group modulus security strength estimates (RFC3526)

| Group  | Modulus  | Strength Estimate 1 | Strength Estimate 2 |
|        |          +----------+----------+----------+----------+
|        |          |          | exponent |          | exponent |
|        |          | in bits  | size     | in bits  | size     |
|  14    | 2048-bit |      110 |     220- |      160 |     320- |
|  15    | 3072-bit |      130 |     260- |      210 |     420- |
|  16    | 4096-bit |      150 |     300- |      240 |     480- |
|  17    | 6144-bit |      170 |     340- |      270 |     540- |
|  18    | 8192-bit |      190 |     380- |      310 |     620- |

Figure 3

Many users seem to be interested in the perceived safety of using larger MODP groups and hashing with SHA2-based algorithms.

7. References

7.1. Normative References

[FIPS-180-4] National Institute of Standards and Technology, "Secure Hash Standard (SHS)", FIPS PUB 180-4, August 2015.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC3526] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP) Diffie-Hellman groups for Internet Key Exchange (IKE)", RFC 3526, DOI 10.17487/RFC3526, May 2003.
[RFC4253] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) Transport Layer Protocol", RFC 4253, DOI 10.17487/RFC4253, January 2006.

7.2. Informative References

[I-D.josefsson-ssh-curves] Adamantiadis, A. and S. Josefsson, "Secure Shell (SSH) Key Exchange Method using Curve25519 and Curve448", Internet-Draft draft-josefsson-ssh-curves-04, March 2016.
[MFQ-U-OO-815099-15] "National Security Agency/Central Security Service", "CNSA Suite and Quantum Computing FAQ", January 2016.
[NIST-SP-800-131Ar1] Barker, and Roginsky, "Transitions: Recommendation for the Transitioning of the Use of Cryptographic Algorithms and Key Lengths", NIST Special Publication 800-131A Revision 1, November 2015.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange (IKE)", RFC 2409, DOI 10.17487/RFC2409, November 1998.
[RFC3174] Eastlake 3rd, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)", RFC 3174, DOI 10.17487/RFC3174, September 2001.
[RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For Public Keys Used For Exchanging Symmetric Keys", BCP 86, RFC 3766, DOI 10.17487/RFC3766, April 2004.
[RFC4419] Friedl, M., Provos, N. and W. Simpson, "Diffie-Hellman Group Exchange for the Secure Shell (SSH) Transport Layer Protocol", RFC 4419, DOI 10.17487/RFC4419, March 2006.
[RFC4432] Harris, B., "RSA Key Exchange for the Secure Shell (SSH) Transport Layer Protocol", RFC 4432, DOI 10.17487/RFC4432, March 2006.
[RFC4462] Hutzelman, J., Salowey, J., Galbraith, J. and V. Welch, "Generic Security Service Application Program Interface (GSS-API) Authentication and Key Exchange for the Secure Shell (SSH) Protocol", RFC 4462, DOI 10.17487/RFC4462, May 2006.
[RFC5656] Stebila, D. and J. Green, "Elliptic Curve Algorithm Integration in the Secure Shell Transport Layer", RFC 5656, DOI 10.17487/RFC5656, December 2009.
[RFC6194] Polk, T., Chen, L., Turner, S. and P. Hoffman, "Security Considerations for the SHA-0 and SHA-1 Message-Digest Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011.
[safe-curves] Bernstein, and Lange, "SafeCurves: choosing safe curves for elliptic-curve cryptography.", February 2016.

Author's Address

Mark D. Baushke Juniper Networks, Inc. 1133 Innovation Way Sunnyvale, CA 94089-1228 US Phone: +1 408 745 2952 EMail: URI:

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